In order to support a high-data-rate, low-latency, packet-optimized wireless system with increased coverage and capacity, a long term evolution (LTE) of the third generation (3G) system, (i.e., evolved universal terrestrial radio access (E-UTRA)), is currently being developed. In LTE, orthogonal frequency division multiple access (OFDMA) and single carrier frequency division multiple access (SC-FDMA) are proposed air interface technologies to be used in the downlink and uplink transmissions, respectively.
Meanwhile, a wideband code division multiple access (WCDMA)-based evolution of the conventional universal mobile telecommunication system (UMTS), (i.e., evolved high speed packet access (HSPA+)), is also proposed.
Use of multiple antennas at a base station and/or a wireless transmit/receive unit (WTRU) enhances system performance. In the downlink, specifically, these antennas can be used to provide transmit diversity and/or beamforming. The combination of multiple transmit and receive antennas may increase throughput without the need for additional power or bandwidth. There are two main multiple antenna technologies, namely beamforming and MIMO. Both are proposed to be used in LTE and HSPA+ systems as well as other wireless systems such as a wireless local access network (WLAN).
A base station generates a steering matrix for transmit beamforming. In order for the base station to calculate an appropriate steering matrix to a particular WTRU, the base station needs to have an accurate estimate of the channel state information (CSI) to that WTRU. However, differences in the radio frequency (RF) drift and impairments between several transmission radios of a multiple-antenna transmitter may cause severe degradation in performance of transmit beamforming and MIMO. Therefore, calibration should be performed to ensure adequate performance.
In a time division duplex (TDD) mode, differences in transmit and receive chains in a base station and a WTRU destroy the inherent reciprocity of the wireless channel. Thus, the main purpose of RF calibration in a TDD mode is to remove the differences in transmit and receive chains and enhance reciprocity in the observed baseband-to-baseband channels. These differences lead to differing calibration approaches.
Currently, there is no control and signaling procedure to support channel state estimation and RF calibration in LTE or HSPA standards when transmit beamforming is used.